Django form choices disappear on page refresh

I have a Django form that has a select field to choose a User for a ForeignKey relationship. Initially when the page loads it displays the select field with all Users as options. When refreshing the page (or reloading as a result of validation errors) the select field has no options. Continuing to refresh loads them sometimes but not consistently.

forms.py:

user_choices = ((obj.id, obj.email) for obj in User.objects.all())

class EditContentForm(forms.ModelForm):
    class Meta:
        model = Content
        fields = ['title', 'user']

    def __init__(self, *args, **kwargs):
        super(EditContentForm, self).__init__(*args, **kwargs)
        self.fields['title'].widget = TextInput(attrs = {'class' : 'form-control'})
        self.fields['user'].widget = Select(attrs = {'class' : 'form-control'}, choices = user_choices)

I've used Django forms to do this many times and never seen this issue. Any ideas are appreciated, thank you.